Don't count on Uncle Sam or your boss. Take control of your retirement.

My life savings is worth $____, and I can reasonably expect it to be worth $____ when I retire.

In a past issue of the Motley Fool Rule Your Retirement newsletter, I introduced readers to a couple who retired in 1991 at age 38 and traveled the world year-round. How did they do it? No, they aren't Rockefellers, and they didn't get millions of stock options working for Microsoft(NASDAQ:MSFT). They crunched the numbers, analyzed their expenses, and realized they could live off their current assets. In other words, they could fill in those blanks.

If you're like most people, you want to retire one day, whether that means a life of complete leisure, a full-time devotion to a cause you're passionate about, or a whole new career (based on your interests, on your terms).

However, if you do fall into that "most people" category, you haven't broken that goal of retirement down to dollars. Less than half of working Americans have calculated how much they'll need in retirement and what kind of income they can expect.

What will support your retirement?Social Security and traditional pensions make up two legs of the proverbial three-legged stool that most of us will use to support our retirements. For many people, these programs provide the overwhelming majority of their post-work income. In fact, approximately two-thirds of retirees rely on Social Security for at least half of their income.

But these people aren't living the retirement you want. The average Social Security benefit is just $11,000 a year. And, chances are, your pension (if you have one) won't provide much more -- unless you're a retired CEO such as Kent Kresa of Northrop Grumman(NYSE:NOC) (who will receive $2.7 million annually for life), Treasury Secretary and former CSX(NYSE:CSX) honcho John Snow ($2.5 million annually due to getting almost twice as much service credit), or General Electric's(NYSE:GE) Jack Welch (who gets $9 million annually).

No, for the retirement you want, you'll have to focus on the third leg: personal savings.

Make or break your retirementOf the three legs, this is the Fool's favorite. The other two are largely out of our hands. Sure, we can become better educated about Social Security and elect officials who might make real changes, and we can choose employers who offer defined-benefit plans and a reasonable expectation that they'll honor their obligations. But this amount of control pales in comparison to how much our retirement is improved by aggressive saving and smart investing.

A recent study by Hewitt Associates(NYSE:HEW) found that, for people who have access to a 401(k), the plan will provide more than half of their retirement income. Save a little more, invest wisely, and keep costs down, and you could have even more retirement fun money. It's up to you. It's your choice.

Unfortunately, many people are making other choices. Check out these stats:

The percentage of eligible workers who participated in a 401(k) plan declined from 75% to 72.4% last year, the first drop in 20 years (Plansponsor.com).

The percentage of all workers who regularly save for retirement has dropped to 42%, the lowest rate since 1980 (RoperASW).

The same percentage of workers (42%) cash out their 401(k)s rather than transfer the assets to an IRA or a new employer's retirement plan, smashing their nest egg and possibly triggering taxes and penalties (Hewitt Associates).

Three-fourths of workers age 55 to 64 have less than $56,000 saved (Congressional Research Service).

Add a declining savings rate to a growing debt level, and many Americans will find it very difficult to retire.

What this means to youThose are not encouraging stats, to be sure. But those are national, average numbers. The numbers that really matter to you are the ones that will fill in those blanks. How can you crunch those numbers? You can start by gathering information about your assets and using some online calculators to make the estimates. Visit our newly revamped Retirement Area for information about tax-advantaged accounts and common pitfalls. Give the Motley Fool Rule Your Retirement service a 30-day free trial, which includes a monthly newsletter and a free copy of Money After 40: Building Wealth for a Better Life, the recent best-seller by Motley Fool co-founders Tom and David Gardner. We'll help you figure out where you stand now, where you want to go, and what you have to do to get there.

The only way you'll have the retirement you want is by taking control. I can guarantee that your prospects will improve because you will learn what you have to do to accomplish your goals. And you're far better off taking necessary steps sooner rather than later -- when it might be too late.

Robert Brokamp, editor ofMotley Fool Rule Your Retirement,has a good retirement plan, but his kids keep drawing on it. He does not own any of the companies mentioned in this article. For a one month-trial subscription to Rule Your Retirement, click here. For a limited time, we'll throw in a bonus copy of Tom and David Gardner's book, Money After 40: Building Wealth for a Better Life, just for trying.The Motley Fool is investors writing for investors.